This invention relates to a photopolymerization initiator having as its active component a specific benzophenone group-containing polyperoxyester and to a method for the polymerization of a radically polymerizable unsaturated compound by the use of the aforementioned photopolymerization initiator.
In polymerizing and setting photosensitive substances, the photopolymerization method and the photosetting method are advantageous over the thermopolymerizing method, the thermosetting method, and the oxidative setting method in respect that they permit polymerization and setting to be effected rapidly at low temperatures, offer high productivity, save energy, and avoid causing environmental pollution. Further, they are capable of providing selective setting. Owing to these numerous merits, these methods have found extensive utility in the production of printing inks, paints, adhesive agents, printing plates, and printed boards.
Various types of polymerization initiators have been developed for use in the photopolymerization method and the photo-setting method.
For example, such photopolymerization initiators as benzoin, benzoin ethers, benzil, allyldiazonium salts, benzophenone derivatives, acetophenone derivatives, xanthates, thioxanthones, and halogenated hydrocarbons which generate radicals under the influence of ultraviolet rays have been known to the art (Journal of Oil and Color Chemists Association, Vol. 59, pp 166-170 (1976)).
It has also been known that such organic peroxides as benzoyl peroxide and di-t-butyl peroxide are usable as photopolymerization initiators. These organic peroxides generally absorb only the portion of light not exceeding 320 nm (Chemical Review, Vol. 68, pp 125-151 (1968)). Combined use of such organic peroxides with a sensitizer has been tried. For example, combinations of such organic peroxides as t-butyl peroxybenzoate and lauroyl peroxide with such photosensitizers as naphthalene, 1-naphthylaldehyde, and naphthylphenyl ketone have been proposed (U.S. Pat. No. 4,171,252). The elaborate incorporation of a sensitizer proves disadvantageous from the operational point of view. Moreover, such combinations entail the problem that the transfer efficiency of light energy from sensitizers to organic peroxides is low and the photodecomposition efficiency of organic peroxides is low (Journal of American Chemical Society, Vol. 93, pp 7005-7012 (1971)). As a solution to this problem, adoption of organic peroxides of high photodecomposition property was tried. For example, U.S. Pat. No. 4,171,252 discloses use of 1-t-butylperoxycarbonyl naphthalene and Journal of Organic Chemistry, Vol. 44, pp 4123-4128 discloses use of 4-t-butylperoxycarbonyl benzophenone for photopolymerization of methyl methacrylate. These compounds, however, have been found to exhibit low polymerization activity.
Among the photopolymerization initiators so far accepted for practical use are counted Michler's ketone and thioxanthones. Michler's ketone has been reported as possessing a carcinogenic property (Chemical Abstract, Vol. 92, 123109Z) and thioxanthones have poor solubility (Japanese Patent Disclosure No. SHO 57(1982)-23602).
As described above, the photopolymerization initiators developed to date have much room for further improvement.
An object of this invention is to provide a photopolymerization initiator that has no toxicity, possesses high thermal stability and high solubility, and is superior to any of the conventional photopolymerization initiators in terms of practical utility, and a method for polymerization by the use of the aforementioned polymerization initiator.